1. Field of the Invention
The present invention relates to a method of anticorrosive treatment for soft water boilers. More particularly, it relates to a method of anticorrosive treatment for soft water boilers which is convenient to prevent corrosion in a boiler water system using soft water at high temperatures, where a ferrous metal such as iron, mild steel or cast iron is brought into contact with soft water, especially pitting corrosion.
2. Description of the Prior Art
Raw water boilers using raw water such as industrial water, well water or the like (as is), soft water boilers and pure water boilers are generally known, and in many cases the temperature of their boiler water system is set at 110.degree. C.-400.degree. C. under various pressures.
In soft water boilers using water from which hardness ingredients are almost all removed by pretreatment (i.e. soft water), scale troubles due to hardness ingredients are few.
However, anionic ingredients are not removed from such water, and so its corrosive tendency due to anions is rather increased and its pH is lowered.
When the pH is low and the anionic ingredients exist in abundance, the process of pitting corrosion is accelerated, which is the most frequently appearing and the most dangerous one among various corrosions of boilers which is generated locally and deeply, owing mainly to dissolved oxygen in water.
Thus, in boilers using soft water, it has been hitherto effected to protect them from generation of such pitting corrosion by removing the greater part of the dissolved oxygen in boiler feed water by means of a deaerator and further removing the remaining oxygen reductively by pouring an oxygen scavenger agent such as hydrazine, sodium sulfite or the like into the water, and then, after such two-step treatment, adding polyphosphoric acid or orthophosphorates as anticorrosive agent to the water and, if necessary, pouring an alkaline agent into the water, to keep the water at a pH of 10-12. This method is prescribed in Japan as the standard method of anticorrosive treatment for water for soft water boilers (JIS B-8233/1977; hereinafter, called the deoxidation/alkali treatment method).
In the above-mentioned deoxidation/alkali treatment method, however, the phosphates used are those compounds which show an anticorrosive effect owing to the formation of a precipitate film. Accordingly, formation of a fine and firm film cannot be expected and prevention of generation of pitting corrosion over a long period of time is difficult by such treatment method. Further, since it is impossible to add the oxygen scavenger used in combination, such as hydrazine, sodium sulfite or the like, to the water successively in a concentration corresponding accurately to the concentration of the dissolved oxygen in the feed water, the method is usually carried out by adding 1.2-1.5 times the oxygen scavenger per the estimated dissolved oxygen. Then, it sometimes happens that the addition goes to excess or falls short. When the amount added falls short, rust will generate in boilers and, when the amount added goes to excess, ammonia or sodium sulfide will generate due to decomposition of hydrazine or sodium sulfite. Generation of these ingredients is unfavorable, since they may induce corrosion of copper family metals in the vapor system of boilers.
Thus, the deoxidation/alkali treatment method is troublesome in the control of concentration and in the method of addition of each agent to be added and cannot attain a satisfactory anticorrosive effect in many cases. Moreover, hydrazine involves the problem that it has toxicity (cancerogenicity) and it must be dealt with carefully, and sodium sulfite involves the problem that a high concentration of salt, which may be brought about as the boiler water is highly concentrated, causes corrosion and accordingly it is impossible to operate the boiler with highly concentrated boiler water.
On the other hand, a combination of anticorrosive agents developed by one of the inventors of the present invention, namely using a phosphonate, a hydroxycarboxylic acid and a zirconium compound in combination (Japanese Patent Application (OPI) No. Sho 59-16983) and a method of anticorrosion using a molybdate, a citrate, an aminophosphonic acid, an azole compound, etc. in combination (Japanese Examined Patent Publication No. Sho 61-15158) have been known hitherto. However, it was difficult to attain a satisfactory anticorrosive effect by merely applying such anticorrosive agents or such method of anticorrosion to the anticorrosive treatment of iron family metals in high temperature soft water boilers, and it was still necessary for preventing generation of pitting corrosion to effect such a deoxidation treatment as the above-mentioned one and, as the case may be, also an alkali addition treatment.
The present invention is intended circumstances and intends to provide a new method of anticorrosive treatment for soft water boilers, which is convenient to protect the ferrous metals in a soft water boiler system at high temperatures from generation of corrosion, especially of pitting corrosion, without effecting any troublesome deoxidation treatment which requires use of a deaerator and addition of oxygen scavengers.
It is to be noted that, although each of the ingredients (a), (b) and (c), which is described hereinafter, and which is used in the method of the present invention has been known as a general anticorrosive ingredient (U.S. Pat. No. 4,138,353, Japanese Patent Application (OPI) Nos. Sho 48-71335 and Sho 52-103338 etc.), combined use of these three ingredients for soft water boilers has been hitherto unknown.